Due diligence required to quantify and visualise agrichemical spray deposits using dye tracers

Dye tracers allow researchers to optimise spray application equipment and spray formulations by quantifying the amount of agrichemical spray retained and/or quantifying the area covered (including the size and number of spray deposits). This paper discusses the issues involved in selecting a dye fit-for-purpose, and outlines the experimental checks required to ensure the dye will provide accurate, reliable results of spray retention and spray coverage. This is illustrated using a pilot study to identify a dye, or combination of dyes, capable of quantifying both spray retention and spray coverage on the model species Eucalyptus fastigata H. Deane & Maiden. Due diligence in selecting a dye to quantify agrichemical spray retention requires checks to ensure the dye is quantifiable, extractable from the target surface, and stable not only in the spray formulation used, but also under the prevailing environmental conditions (sunshine, temperature and humidity), and after storage on the target surface prior to extraction and analysis. To visually quantify spray coverage and/or the number and size of deposits, tests are required to ensure the dye provides clear contrast between the dye deposits and non-covered surface. Tests are also required to ensure the dye selected does not alter the physical properties of the spray. There are numerous reasons why a dye may fail to provide accurate spray deposit or coverage results, or correct results. In this study, one dye was not representative of the formulation spread (and therefore coverage). Two others could be used to visualise spray deposits but, due to UV degradation, retention was not quantifiable. A fourth dye was too difficult to visualise on the target. A fifth dye (pyranine) was a suitable candidate; it was photo-stable in dried deposits, quantifiable and could be visualised with an intense UV light source. The plant target itself presented unexpected complications; the sampling method previously employed in retention trials enabled pyranine dye to penetrate the leaves, due to moisture from the dehydrating leaf solubilising and mobilising spray dye deposits during storage. A revised sampling method, allowing the leaves to dehydrate without condensation, was required to fully recover the dye. Using the identified methodology pyranine proved to be suitable as a dual-purpose dye capable of quantifying both spray retention and spray coverage under the anticipated experimental conditions. This paper highlights the requirement for due diligence in dye selection prior to investing in a full-scale field trial. Researchers must check every stage in the experimental process of selecting dyes as deposit tracers. If not, they may fail to get results or, more likely, unwittingly publish incorrect results.